Aircraft Tube Bending - In the early days of aviation, copper tubing was widely used in aviation fluids. In modern aircraft, copper pipes are usually replaced by aluminum alloy, corrosion-resistant steel or titanium.
Tubing made from 1100 H14 (1⁄2-hard) or 3003 H14 (1⁄2-hard) is used for general purpose lines with low or negligible fluid pressure, such as instrument lines and vent pipes. Manufactured from 2024-T3, 5052-O and 6061-T6 aluminum alloys, tubing is used in low to medium pressure general purpose systems such as 1000 to 1500 psi hydraulic and pneumatic systems, as well as fuel and oil lines . .
Aircraft Tube Bending
Corrosion-resistant steel tubing, either annealed CRES 304, CRES 321, or CRES 304-1⁄8-hard, is widely used in high-pressure (3,000 psig or greater) hydraulic systems for landing gear, flapper, brake, and fire zones. Higher breaking strength allows the use of pipes with thinner walls; therefore, the final weight of the installation is not much greater than a thick-walled aluminum alloy tube. Steel wire ropes are used where there is a risk of foreign object damage (FOD) (ie undercarriage and undercarriage). Stamped or MS flareless fittings are used with corrosion-resistant pipes. Although identification markings for steel tubing vary, each typically includes the manufacturer's name or trademark, the Society of Automotive Engineers (SAE) number, and the physical condition of the metal.
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3AL-2.5V titanium tubing and fittings are widely used in transportation grade and high performance aircraft hydraulic systems for pressures above 1,500 psi. Titanium is 30 percent stronger than steel and 50 percent lighter than steel. Cryofit fittings or stamped fittings are used with titanium tubing. Do not use titanium tubing or fittings in any oxygen system. Titanium and titanium alloys are reactive with oxygen. When the newly formed titanium surface is exposed to gaseous oxygen, spontaneous combustion can occur at low pressure.
Before any flight pipe repairs are made, it is important to accurately determine the pipe material. Aluminum alloy, steel, or titanium tubing can be easily identified by appearance when used as the main tubing material. However, it is difficult to tell whether the material is carbon steel or stainless steel, or whether it is aluminum alloy 1100, 3003, 5052-O, 6061-T6, or 2024-T3. To accurately identify the material that was used in the original installation, compare the code marking on the replacement tube to the original marking on the replacement tube.
On large aluminum alloy pipes, the alloy designation is embossed on the surface. On small aluminum tubes, the indication can be stamped on the surface; but more commonly it is indicated by a color code, no more than 4" wide, painted on the two ends and about midway between the ends of some tubes. If the strip is two colors, half the width is used for each color [Figure 1]
If the code marking is difficult or impossible to read, it may be necessary to test material samples for hardness using hardness tests.
Tube And Pipe Bending 101
Metal pipes are defined by their outside diameter (OD), which is measured in sixteenths of an inch. For example, a number 6 pipe is 6⁄16" (or 3⁄8"), a number 8 pipe is 8⁄16" (or 1⁄2"), and so on. The pipe diameter is printed on all solid pipes. In addition to other classifications, pipes are available in different wall thicknesses as a means of identification. Therefore, when installing pipes, it is important to know not only the material and outer diameter, but also the wall thickness. Wall thickness is printed on the pipe in thousandths of an inch. To determine the inside diameter (ID) of the pipe, subtract twice the wall thickness from the outside diameter. For example, a piece of #10 tubing with a wall thickness of 0.063" has an inside diameter of 0.625" - 2(0.063") = 0.499".
Damaged pipes and fluid lines should be repaired with new parts if possible. Unfortunately, sometimes replacement is impractical and repair is necessary. Scratches, scuffs or minor corrosion on the outside of the fluid lines can be considered minor and can be smoothed out with a polishing tool or aluminum wool. Limits on the amount of damage that can be repaired in this way are discussed on this page in the Solid Pipe Inspection and Repair section. If a liquid line assembly needs to be replaced, the fittings can often be salvaged; then the repair involves only forming and replacing the tube.
Pipe forming consists of four processes: cutting, bending, flaring and beading. If the tube is small and made of a soft material, the assembly can be formed by hand bending during installation. If the pipe is 1⁄4" or larger in diameter, bending by hand without tools is impractical.
When cutting pipes, it is important to create a square end without burrs. Pipes can be cut with a pipe cutter or hacksaw. The cutter can be used with all soft metal pipes such as copper, aluminum or aluminum alloy. The proper use of a pipe cutter is shown in Figure 2. Special chip-free cutters are available for cutting 6061-T6 aluminum, stainless steel, and titanium pipe.
Vr3 Engineering: Precision Tube Fabrication
The new piece of tubing should be cut about 10 percent longer than the tubing to be replaced to allow for slight changes in bend. Place the tubing in the cutting tool with the cutting wheel where the cut is to be made. Rotate the cutter around the tube by applying slight pressure to the cutting wheel while turning the thumbscrew. Too much pressure on the cutting wheel at the same time can distort the tube or cause excessive burrs. After cutting the tube, carefully remove all burrs from the inside and outside of the tube. Use a knife or the edge of a pipe cutter. The deburring operation can be performed using a deburring tool. [Figure 3] This tool is capable of removing both inner and outer springs by simply turning the end of the arc.
When performing the deburring operation, be very careful not to reduce the end wall thickness of the pipes and break them. Very mild damage of this type can result in broken flares or defective flares that do not seal properly. Use a fine tooth file to file the edge square and smooth.
If a pipe cutter is not available, or if the pipes are cut from hard material, use a fine-toothed hacksaw, preferably 32 teeth per inch. The use of a saw reduces the amount of pipe deformation during the cutting operation. After sawing, square the end of the pipe and smooth it, removing all burrs.
An easy way to hold small diameter pipe while cutting is to place the pipe in a combination flaring tool and clamp the tool in a bench. Make a cut about half an inch from the torch tool. This procedure minimizes saw vibration and prevents damage to the tube if it is accidentally hit with the hacksaw frame or file handle while cutting. Make sure all sawdust and plugs are removed from the tube.
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The purpose of pipe bending is to obtain a smooth bend without flattening the pipe. Tubing less than 1⁄4" in diameter can usually be bent without the use of a bending tool. For larger sizes, portable hand or industrial benders are commonly used. Figure 4 shows the preferred methods and standard bend radii for bending tubing by pipe size.
Using a hand bender, insert the tube into the groove of the bender so that the measured end is to the left of the mold block. Line up the two zeros and line up the mark on the tube with the L on the shaped handle. If the measured end is on the right side, line up the mark on the tube with the R on the shaped handle. Pull the form handle in a steady motion until the zero mark on the form handle lines up with the desired bend angle as shown on the radius block. [Figure 5]
Hand benders come in a variety of sizes to suit the diameter of the pipe. Make sure you select the correct bender for the correct pipe diameter. Figure 6 shows manual benders available for various pipe sizes. As a rule, the size of the pipe is punched into the bender. [Figure 7]
Bend the tubing carefully to avoid excessive flattening, kinks, or creases. A small amount of flattening in the bends is permissible, but the minor diameter of the flattened portion shall not be less than 75 percent of the original outside diameter. Do not install pipes with flat, wrinkled or irregular bends. Crimped bends are usually the result of trying to bend thin-walled pipes without using a pipe bender. Excessive flattening causes fatigue failure of the tube. Examples of correct and incorrect pipe bends are shown in Figure 8.
Husky Tube Bending Tool 80 535 111
Pipe bending machines for all types of pipes are commonly used in repair stations and large maintenance shops. With such equipment, good bends can be made on large diameter pipes and on pipes made of hard material. A CNC™ production tube bender is an example of this type of machine. [Loik
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